Commonly meta-aramid polymers useful for spinning fiber are obtained from the reaction, in a solvent, of a dime and a diacid chloride, typically isophthaloyl chloride. This reaction produces hydrochloric acid as a by-product. Generally in manufacturing, this acid by-product is neutralized by the addition of a basic compound to form a salt. Depending on the selection of the basic compound and the polymerization solvent, the salt formed on neutralization may be insoluble in the polymer solution and therefore precipitate out of the solution, or the salt may be soluble as a salt-polymer and/or salt solvent complex. Thus, spinning solutions are known which range from salt-free to having a relatively high concentrations of salt. For example, if no salt is removed from the typical meta-aramid, base neutralized polymerization reaction solution (approximately 20% by weight polymer solids), the salt concentrations in the polymer solution may be as high as 9% by weight.
There is an advantage to directly spin polymer synthesis solutions containing high concentrations of salt. Although salt content is known to be beneficial in the spinning solution as a means to increase polymer solution stability, the wet spinning of meta-aramid polymer from solutions containing concentrations of three percent (3%) or more by weight salt has generally resulted in fibers having poor mechanical and other physical properties. In practice wet spinning of meta-aramid fibers having acceptable physical properties was accomplished from salt-free polymer solutions or from polymer solutions containing low concentrations of salt. Polymer solutions containing low concentrations of salt are those solutions that contain no more than 3% by weight salt. There are teachings of wet spinning processes from high salt containing solutions, but in order to develop acceptable mechanical properties in the fibers produced from these processes, the fiber must be subjected to a hot stretch.
In one method to produce a low salt spinning solution, the polymerization is carded out with at least two additions of the diacid chloride. The polymerization is initiated by the addition of an mount of the diacid chloride that is less than required for complete polymerization of the diamine. Anhydrous ammonia is typically added to this polymerization reaction solution while the solution viscosity is still low enough to allow the separation of a solid phase from the solution. The anhydrous ammonia neutralizes the hydrochloric acid that has formed as a result of the polymerization, forming ammonium chloride, which is insoluble in the polymer solution and may be removed. Additional diacid chloride may then be added to the reaction solution to complete the polymerization. Acid resulting from this second phase of polymerization may be neutralized producing a low concentration of salt in the polymer solution that is used for spinning.
Salt-free polymer can be made by removal of hydrochloric acid from the reaction solution or by the removal of salt from a neutralized reaction mixture, but the processing requires a number of steps and additional economic investment. Salt-free spinning solutions may be spun without the addition of salt, or salt can be added to some specifically desired concentration.
As noted above, prior art taught wet spinning processes for low salt and even high salt containing spin solutions; however, these processes required hot stretches to provide a product with acceptable mechanical properties. In particular, some substantial mount of hot stretching and fiber crystallization was required in these processes to provide mechanical integrity to these wet spun fibers.
The hot stretching necessary to develop mechanical properties in the fibers also causes limitations in fiber use. It is known in the art of spinning aramid fibers that exposing the fiber to temperatures at or near the polymer glass transition temperature, produces some degree of crystallization. While crystallizing the fiber improves certain physical and mechanical properties, it causes the fiber to be especially difficult to dye. These crystallized (hot stretched), difficult to dye fibers are limited in their use in textile applications. Until the development of the present invention, it has not been possible to produce wet spun meta-aramid fibers having excellent physical properties and improved dyeability.
The difficulty in producing meta-aramid fibers from wet spinning of salt-containing spin solutions is evident in the earlier patent literature. For example, U.S. Pat. No. 3,068,188 to Beste, et al. suggested that fibers could be spun by either wet or dry spinning processes, but did not disclose any process for wet spinning. Fibers produced by wet spinning polymer solutions containing high concentrations of salt were generally characterized by the presence of large voids. These voids affected the ability of the fiber to be effectively drawn. On drawing, void-containing fibers were not only subject to a greater degree of fiber breakage, but those fibers that were successfully drawn developed mechanical properties which were much lower than the properties that could be developed in dry spun fibers or in fibers which were wet spun salt-free polymer solutions. Dry spinning and wet spinning from salt free polymer solutions are methods known to produce fibers that are free of large voids.
The deficiencies of fibers produced by wet spinning before the process of the present invention are evidenced by U.S. Pat. No. 3,414,645 to Morgan which taught the advantages of the air-gap (dry-jet wet) spun, void-free fiber over that of a wet spun fiber; by U.S. Pat. No. 3,079,219 to King which taught that a calcium thiocyanate containing coagulation bath was required to improved the strength and produce serviceable wholly aromatic, wet spun polyamide fibers and by U.S. Pat. No. 3,642,706 to Morgan which taught the incorporation of a wax into the polymer spinning solution to improve physical properties of wet spun meta-aramid fiber.
Staged wet draws combined with hot stretching was taught in U.S. Pat. No. 4,842,796 to Matsui et al. for fibers produced primarily from salt-free spinning solutions. Japanese Pat. Publication Kokai 48-1435 and Kokai Sho 48-19818 taught the combination of certain salt/solvent ratios in the coagulation bath coupled with hot fiber stretches to crystallize the fiber. Japanese Patent Publication Kokolm Sho 56-5844 taught the combination of two coagulation baths to exhaust solvent from the fiber followed by conventional drawing and hot stretch crystallization to produce suitable wet spun fiber from polymer spinning solutions having high salt concentrations.
The present invention provides a process by which polymer solutions rich in salt may be wet spun and fully wet drawn in a single stage to achieve desirable and useful mechanical properties without the need of a hot stretch and fiber crystallization. The fiber produced by the present process is more easily dyed to deep shades. The fiber made from the process of the present invention may, optionally, be heat treated and crystallized to produce properties required for industrial and other high performance applications.